1. Field of the Invention
This invention relates to the field of motion-picture and television video. More specifically, without limiting the generality of the invention, this invention relates to the field of video assist systems for motion-picture cameras.
2. Description of Related Art
In the course of making a motion picture, it is often advantageous to duplicate the image which is available to the motion-picture camera operator (the "operator") and present this image simultaneously to another person. The operator controls the placement and operation of the motion-picture camera, and is generally the only person who sees the image captured by the camera before that image is actually developed from film. If the operator fails to notice extraneous elements in the image, or fails to properly position the camera, the image will be of poor quality and the film may have to be re-shot.
A "video assist" camera duplicates the image which is available to the operator, typically by use of a television ("TV") camera. The use of a TV camera is advantageous because it allows a signal representing the TV image (a "TV signal") to be electronically routed to remote points, and to be electronically manipulated by well known TV techniques, without much additional work. Coordination of the TV image with that of the motion-picture image has been subject to the problem that the TV image will tend to flicker, because the rate at which the motion-picture camera captures its image is not synchronized with the rate at which the TV image is displayed.
A motion-picture camera typically captures its image by exposing film at a particular fixed rate (its "capture rate"); in many motion-picture films this rate is about 24 frames/sec. In motion-picture cameras using a rotating mirror or reflex principle, the film and viewfinder images are blanked out alternately at the same 24 frame/sec capture rate. A TV camera operates with a capture rate which may be completely different, and is typically 50 or 60 frames/sec in common TV systems. (As used herein, "frame" means a single two-dimensional display image, and specifically means a single TV "field"; a TV "frame" comprising a pair of interlaced fields is referred to herein as a "frame pair".) Because these rates are different, a TV image which is captured at a motion-picture capture rate will tend to exhibit visual "flicker" at the fundamental and beat frequencies.